Flying for maximum range – propeller propulsion

Whether in war or peace, we shall often wish to use an aircraft to best advan­tage for some particular purpose – it may be to fly as fast as possible, or as slowly as possible, or to climb at maximum rate, or to stay in the air as long as possible, or, perhaps, most important of all, to achieve the maximum dis­tance on a given quantity of fuel. Flying for maximum range is one of the outstanding problems of practical flight, but it is also one of the best illustra­tions of the principles involved. To exploit his engine and aircraft to the utmost in this respect, a pilot must be not only a good flier, but also an intel­ligent one.

The problem concerns engine, propeller and aircraft; it also concerns the wind. In this book we are interested chiefly in the aircraft, but we cannot solve this problem, and indeed we can solve few, if any, of the problems of flight, without at least some consideration of the engine and the propeller, or jet, or rocket, or whatever it may be, and how the pilot should use them to get the best out of his aircraft. As for the wind, we shall, as usual in this subject, first consider a condition of still air.

The object in any engine regardless of type is to burn fuel so as to get energy and then to convert this energy into mechanical work. In order to get the greatest amount of work from a given amount of fuel, we must, first of all, get the maximum amount of energy out of it, and then we must change it to mech­anical work in the most efficient way. Our success or otherwise will clearly depend to some extent on the use of the best fuel for the purpose, and on the skill of the engine designer. But the pilot, too, must play his part. To get the most heat from the fuel, it must be properly burned; this means that the mixture of air to fuel must be correct. In a piston engine, what is usually called ‘weak mixture’ is, in fact, not so very weak, but approximately the correct mixture to burn the fuel properly. If we use a richer mixture, some of the fuel will not be properly burned, and we shall get less energy from the same amount of fuel: we may get other advantages, but we shall not get economy. Both the manifold pressure and the revolutions per minute will affect the efficiency of the engine in its capacity of converting energy to work. The problem of the best combination of boost and rpm, though interesting, is outside the scope of this book and at this stage, too, the principles of the recip­rocating and turbine engine begin to differ, while the rocket or ramjet has not got any rpm! For the reciprocating engine we can sum up the engine and pro­peller problem by saying that, generally speaking, the pilot will be using them to best advantage if he uses weak mixture, the highest boost permissible in weak mixture, and the lowest rpm consistent with the charging of the elec­trical generator and the avoidance of detonation. All this has assumed that he has control over such factors, and that the engine is supercharged and that the propeller has controllable pitch. Without such complications, the pilot’s job will, of course, be easier; but the chances are that, whereas a poor pilot may get better results, the good pilot will get worse – far, far worse.

Before we leave the question of the engine and propeller, let us look at a problem which affects all engines in which fuel is burnt to give mechanical energy – not just piston engines.

The problem is that we cannot convert all the energy produced by burning the fuel into mechanical work, however hard we try. What is more, although in a sense we are always trying to do this (and then call the engine inefficient because we do not succeed!) we know quite well why we cannot and never will do it – it is just contrary to the laws of thermodynamics, the laws that govern the conversion of energy into mechanical work. All we can get, even in the best engines and in the hands of the best pilots, is something like 30 per cent of this figure. From each litre of fuel we can expect to get about 31 780 000 joules of thermal energy and hence only 0.3 X 31 780 000 joules i. e. about 9 500 000 joules or newton metres of mechanical energy.

This is what the engine should give to the propeller; and we may lose 20 per cent of it due to the inefficiency of the propeller, and so the aircraft will only get about 80 per cent of 9 500 000, i. e. about 7600 000 joules, or newton metres.

It still seems a large figure – it is a large figure – but, as we shall see, it will not take the aircraft very far. However at this stage we are not so much con­cerned with the numerical value, as with the unit, and to think of it in the form of the newton metre. We have found that a litre of fuel, if used in the best poss­ible way (we have said nothing about how quickly or slowly we use it) will give to the aircraft so many newton metres. Suppose, then, that we want to move the aircraft the maximum number of metres, we must pull it with the minimum number of newtons, i. e. with minimum force. That simple principle is the essence of flying for range.

Tet us examine it more closely. It means that we must fly so that the pro­peller gives the least thrust with which level flight is possible. Feast thrust means least drag, because drag and thrust will be equal.